1. Dihybrid Punnett Squares
What exactly is a dihybrid cross?
Predicting the outcomes from crossing 2 traits.
Why do we use them?
Instead of doing to monohybrid crosses, you can combine them to see all the possible combinations from that particular mother and father.

2. All possible gamete combinations
Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round r wrinkled Y yellow y green
RrYy x RrYy
RY Ry rY ry
RY Ry rY ry
All possible gamete combinations

3. Dihybrid Cross RY Ry rY ry RY Ry rY ry

4. Dihybrid Cross RY Ry rY ry Round/Yellow: 9 Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1 phenotypic ratio
RRYY
RRYy
RrYY
RrYy
RRyy
Rryy
rrYY
rrYy
rryy

5. Incomplete Dominance
When the heterozygous condition is intermediate between dominant and recessive
Happens because one allele is defective, example: R=red flowers, R’= white flowers so RR’ = pink flowers (R codes for enzyme to produce red pigment, R’=no enzyme)

6. RR x R’R’
Incomplete dominance
R’R’ x R’R’
Red
White
F1 generation: 1 Red: 2 Pink: 1 white

7. Codominance Heterozygote expresses both alleles’ conditions
Ex. A black rooster bred with a white hen produces a black and white checkered chicken.
Usually uses both capital letters (black= B, white=W , checkered = BW)

8. Codominance practice: cross a black rooster with a white hen
Key:
B = black
W = white
BW = checkered W W BW BW
Results:
100% checkered (BW) BW BW B

9. Now let’s cross a checkered hen with a checkered rooster.B W
Results:
25% Black
50% Checkered
25% White
(1:2:1) BB BW B BW WW W

10. Codominance
Two alleles are expressed (multiple alleles) in heterozygous individuals.
Example: blood type
1. type A = IAIA or IAi
2. type B = IBIB or IBi
3. type AB = IAIB
4. type O = ii

11. Blood type quick facts Red blood cells are called erythrocytes
Proteins on their surfaces are called antigens, controlled by genes
Antigens make antibodies to foreign substances, which includes RBCs with different antigens on their surface
4 phenotypes: A, B, AB, O
3 alleles: IA, IB, i

12. Surface molecule A is produced.
Phenotype A
Surface molecule A
The lA allele is dominant to i, so inheriting either the lAi alleles or the lA lA alleles from both parents will give you type A blood.
Surface molecule A is produced.

13. Phenotype B The lB allele is also dominant to i.
Surface molecule B
The lB allele is also dominant to i.
To have type B blood, you must inherit the lB allele from one parent and either another lB allele or the i allele from the other.
Surface molecule B is produced.

14. The lA and lB alleles are codominant.
Phenotype AB
Surface molecule B
The lA and lB alleles are codominant.
If you inherit the lA allele from one parent and the lB allele from the other, your red blood cells will produce both surface molecules and you will have type AB blood.
Surface molecule A

15. Phenotype O
No antigens produced

16. A B A or O A or AB B or O B or AB AB A and B None A, B, AB, O O both
Blood Group
Antigens
Antibodies
Can receive from
Can give to A B
A or O
A or AB
B or O
B or AB AB
A and B
None
A, B, AB, O O
both

17. Codominance Problem
Example:homozygous male Type B (IBIB) x heterozygous female Type A (IAi) IB IA i
IAIB
IBi
1/2 = IAIB
1/2 = IBi

18. Another Codominance Problem
Example: male Type O (ii) x female type AB (IAIB) i IA IB
IAi
IBi
1/2 = IAi
1/2 = IBi